The present invention relates to a liquid crystal drive device used in a liquid crystal display device or the like.
With upsizing of a panel of a recent liquid crystal display device, it has been desirable to improve various performance of a liquid crystal drive device for the recent liquid crystal display device. In order to adapt to the upsizing of the panel and an improvement in image quality, double-speed drive has been used and speeding-up has been required even for the liquid crystal drive device. The liquid crystal display device is equipped with a plurality of liquid crystal drive devices, and the number of the liquid crystal drive devices mounted with the upsizing of its panel is also increasing.
FIG. 7 of Japanese Unexamined Patent Publication No. 2006-050572 (patent document 1) shows a 3-bit string resistor type D/A converter. In the string resistor type D/A converter, the number of elemental devices is doubled and the area is also doubled each time the number of bits for gradation voltages increases by one simply. The patent document 1 describes the invention that can be realized without increasing the number of circuit constituent elements abruptly even when gradation voltages required due to an increase in the number of display colors, multigradation, etc. increase.
As shown in FIG. 10, a liquid crystal display device 1000 is equipped with a large number of liquid crystal drive devices shown in FIG. 7 and disclosed in the above-described patent document 1. Source drivers 1010 have string resistors respectively. The string resistors are respectively supplied with a plurality of reference voltages from a reference voltage generating circuit 1030. The string resistors of the plurality of source drivers 1010 are connected in parallel to the reference voltage generating circuit 1030. It is common that a string resistance value is generally set lower than 10 kΩ. When, however, a wiring area on a substrate for mounting the reference voltage generating circuit 1030 and the like is reduced, each wiring resistance on the substrate becomes very high, so that the string resistance value is affected by the wiring resistance, thus exerting an influence on the display.
FIG. 8 shows a simplified model of a source driver applied to FIGS. 10 and 7. FIG. 9 shows voltage transitions at respective points or places in FIG. 8. Voltages V1 and V2 are supplied from the reference voltage generating circuit 1030. A decoder 830 selects a voltage lying between the voltages V1 and V2 according to image data and outputs each voltage corresponding to the data through an amplifier. Data is outputted from a latch circuit to the decoder 830 according to a Load signal. Here, the op amplifier AMP requires an input capacitance. The input capacitance is generally about 1 pF or so. When 400 outputs exist per source driver, there is a need to charge 400 pF in total through two 5-kΩ resistors in parallel.
A recent liquid crystal display device however requires enhancement of the speed of writing into a liquid crystal due to an increase in frame frequency and an increase in the number of respective outputs. It has also been desired to enhance the speed of charging for the input capacitance of an op amplifier AMP in like manner. When the string resistor is set to 10 kΩ or higher to reduce the influence of the wiring resistance in particular in the case shown in FIG. 8, the RC time constant is expressed in the following equation:2.5 kΩ×400 pF=1.0 μsThus, the time required to perform 90% charging becomes about 3 μs and hence a delay occurs in an output waveform.